Your Big Car Is Killing Me

American cars are getting heavier and heavier. Is that dangerous?

Like Americans themselves, American cars are getting heavier and heavier every year. Our new cars are more efficient, with average fuel economy climbing and carbon-dioxide emissions falling over the course of the past 30 years. But that is not because they are lighter. The average new car weighed 3,221 pounds in 1987 but 4,009 pounds in 2010. Even small-size sedans have packed on the pounds, thanks to more-powerful—if more-efficient—engines, as well as features like nicer seats, more safety features, and more legroom.

We pay a hidden cost for our fat cars. They may be sucking up less gas, slowing the degradation of the environment and the warming of the planet. But they have other "negative externalities" that do not figure into their price tags or day-to-day costs as well—notably, more fatal traffic accidents. The heavier the car, the safer it is for the driver and the more dangerous it is for other vehicles and people on the road. You hardly need a Ph.D. in physics to know that getting in a collision with a Hummer is going to be very bad for the driver of a sedan, let alone a Smart Car, let alone a bicycle. So how much are our fat cars costing us? And does it mean our roads are less safe?

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A working paper released this month by two economists from the University of California, Berkeley, Maximilian Auffhammer and Michael Anderson, tackles the first question, attempting to put a price tag on the fatalities associated with big cars. They studied accident data from eight states, identifying the type and weight of vehicles involved in collisions by their VIN numbers. The researchers confirm that the heavy cars kill. Indeed, controlling "for own-vehicle weight, being hit by a vehicle that is 1,000 pounds heavier results in a 47 percent increase" in the probability of a fatal accident. The chance is even higher if the heavy car is an SUV, pickup truck, or minivan. (Taller vehicles tend to do outsize damage, too.)

The researchers then set out to calculate the value of the "external risk" caused by our heftier vehicles. First, they considered a scenario in which a driver chose between a car with the 1989 model-year average weight of 3,000 pounds or the 2005 weight of 3,600 pounds. The heavier car increased the expectation of fatalities by 0.00027 per car—27 deaths per 100,000 such vehicles. "Summing across all drivers," they write, "this translates into a total external cost of $35 billion per year," using the Department of Transportation's value of a statistical life of $5.8 million. Judging against a baseline in which a driver chose the smallest available car, such as a Smart Cars, the cost is $93 billion per year. The price tag climbs beyond $150 billion per year if you include the cost of pedestrian and motorcyclist deaths and figure in multi-car collisions.

Given the relationship between big cars and bad accidents, it might make sense to make such cars more expensive to buy or drive. You could do this with insurance premiums, or lawsuits. But the economists suggest a gas tax, "because it is simple and because gasoline usage is positively related to both miles driven and vehicle weight." They say it would take a 27-cent-per-gallon gas tax to account for the $35 billion per year in extra costs from heavier cars. (To account for the $150 billion in extra costs would require a tax of more than $1 per gallon.)

But, surprisingly, none of this really means the roads are less safe. Indeed, traffic deaths hit their lowest-ever recorded level last year. In part, that is because of cars' advanced safety features—steel cages, better antilock brakes, improved air bags, stronger engines—and those tend to weigh quite a bit. (Better emergency medicine, more stringent policing, improved road quality, and the aging of the populace also figure in.) And all things being equal, a heavier car is safer than a light one.

The problem is that American roads consist of a mix of heavier and lighter cars, and the biggest danger is when they encounter each other. The authors write that relative weight is what is most dangerous in crashes. The recent vogue for lighter vehicles, driven in part by high gas prices and changing fuel-economy standards, has raised worries about the chance of more collision deaths. One study found that higher fuel-economy standards imposed in the 1980s led to 2,000 additional deaths per year. If Americans suddenly start buying many more ultra-light cars, it is not hard to imagine more deadly accidents as a result.

It's an unattractive choice. We can all buy heavier cars, all be a bit safer, and all pay more for gas. Or some of us can buy heavier cars, pay more for gas, and be safer while others buy lighter cars, pay less for gas, and die more often on the road. As for the third option—we all buy lighter cars, all pay less for gas, and all be safer—good luck trying to sell that to America.

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